A major cause of loss in dairy farming is an infection, known as mastitis, which occurs in an animal's udder. Mastitis is caused by contagious pathogens invading the udder and producing toxins that are harmful to the mammary glands. Generally, mastitis starts in one udder quarter.
Somatic cells, predominantly white cells and epithelial cells, enter the mammary gland as a result of damage to the alveolar lining by infection or chemical irritation. The counting of somatic cells excreted in the milk has become a widely used measure of mammary gland inflammation. The somatic cells can be counted by laborious direct microscopic method on stained milk smears, or the cell numbers can also be estimated by direct chemical tests. Other methods measure milk somatic cells indirectly or by determining the concentration of various by-products of the inflammatory response.
Somatic cell count (SCC), which is the number of white cells per milliliter of milk, increases in the bulk tank as mastitis spreads in the herd. SCC scores are used as an international standard in determining milk's quality and price. Most marketing organizations and regional authorities regularly measure SCC on bulk tank milk and use these scores for penalty deductions and/or incentive payments. High SCC scores indicate the presence of mastitis in the herd, which is reflected in the average score of the bulk tank. The bulk tank SCC is a good indicator of overall udder health and as good means for evaluating the mastitis control program.
It is also a high correlation between the bulk milk SCC and the average of individual animal counts. It is not uncommon for a few problem animals to be responsible for greater than 50% of the somatic cells in the bulk tank, particularly in small herds. It should be noted that animals with high milk production and intermediate SCC levels can have a significantly higher percentage of SCC contribution to the tank score than some high SCC cows with low production. For high quality milk the SCC should be less than 200,000 cells/ml. Acceptable milk has SCC scores from 200,000 to 500,000 cells/ml. For infected animals, milk SCC scores are between 600,000 and 1.2 million cells/ml.
When an animal in the herd becomes infected with infectious pathogens a rapid drop in milk production will be noted within two to three days. A high level of bacteria in an animal causes an increased level of somatic cells in milk. An increased level of somatic cells in milk results in poorer quality milk products, which are harder to process. The prevention procedures at milking are less efficient especially when the mastitis is in a subclinical phase and there are no visible signs of the disease. Special efforts have to be made at each milking to detect subclinical mastitis in individual animals.
SCC may be measured by CMT (California Mastitis Test) by utilizing the difference in the extent of aggregation reaction depending on the number of somatic cells, when a surfactant is added to the milk. Since a BTB reagent is also included for pH measurement, it is used as an evaluation index for mastitis by utilizing the fact that increased vascular permeability and accelerated conflict between leukocytes and bacteria during mastitis results in increased salts such as sodium chloride and potassium chloride in the milk, creating a higher alkalinity, and causing a color change from yellow to green and then to blue. The advantages of this measurement are that it can be easily performed by anyone, it can generally distinguish between the presence and absence of mastitis, and it is an extremely low-cost method. The drawbacks of CMT are that diagnosis is difficult until the reaction has occurred, involving the conflict between leukocytes and the bacteria, or after promotion of vascular permeability, and that diagnosis depends on subjective human judgment, so that this method can only serve as an approximate diagnosis method. Diagnosis has been particularly rough in cases where the milk somatic cell count is 300,000/ml or less. The method is thus not suitable to be automated.
Measuring CL (chemiluminescence) activity has also been used for determining the SCC, see e.g. U.S. Pat. No. 6,297,045. A related method is to add to the milk a fluorescent additive, which is absorbed by the cells. By illuminating the milk with light of a particular wavelength the cells will emit a fluorescent light of another characteristic wavelength. By a suitable filter, which filters out light of the characteristic wavelength, the number of cells can be counted.
Such an approach requires that milk samples are taken, that a suitable amount of fluorescent additive has to be added and mixed with the milk, and that particular light sources and filters are used. This is a labor intense and costly procedure. If the method is automated in a milking robot system, particular provisions have to be taken in order to obtain and separate small amounts of milk, which is representative of the milk from a cow or an udder of a cow.
Mastitis may alternatively be detected by measuring changes in the electrical conductivity of milk as generally, ion concentration, and thus electrical conductivity, in mastitic milk is higher than in normal milk. Electrical conductivity is generally measured with a DC or AC circuit having a probe positioned in the flow of milk. The most sensitive part of this on-line method is the probe. The probe generally includes two electrodes to which an AC or DC current is supplied to create an electrical circuit through the milk. The conductivity of the milk is evaluated by measuring the current variations in the circuitry that includes the probe. However, the readings are often inaccurate due to deposits of colloidal materials from the milk on the electrodes, and also due to polarization. Polarization occurs because some of the ions migrating towards the electrodes are not neutralized and consequently, an offset, or leakage current is generated between the electrodes. The presence of the leakage current results in inaccurate conductivity readings. Different aspects on milk conductivity measurements have been patented, see e.g. U.S. Pat. Nos. 3,762,371; 5,416,417; 5,302,903; 6,307,362 B1; and 6,378,455 B1.
Conductometry has disadvantages in that it depends on changes occurring by inflammation reaction after the bacteria invade and conflict with the leukocytes, and therefore it is unsuitable for diagnosis in the initial stages of mastitis, while it has poor reproducibility due to substantial differences in electrolyte components and concentrations in different teats or different cows even with normal milk, such that diagnosis is risky by this diagnostic method alone.
Another potential problem using milk conductivity measurements to discover mastitis is that the conductivity of the milk is heavily dependent on the milking intervals, see Influence of different milking intervals on electrical conductivity before alveolar milk ejection in cows, K. Barth and H. Worstorff, Milchwissenschaft 55(7), 2000, p. 363. Thus, the milking intervals have to be taken into consideration if milking times are not as fixed as in conventional milking systems.